The present invention relates, in general, to methods for fabricating monolithic three-dimensional structures on a substrate, and more particularly to methods for fabricating optical couplers for integrated laser and waveguide structures.
Advances in the technology available for processing and fabricating semiconductor devices have allowed structures of various shapes to be formed on the surface of a wafer, as by the use of a variety of photosensitive materials applied to the surface of the substrate and various photolithographic processes for defining structures to be fabricated. For example, conventional photoresist materials can be spun onto a substrate surface and then exposed to light in specified regions, as through the use of photolithographic masks, to create patterns on the substrate after the photoresist has been developed. Such techniques may be used, for example, to fabricate integrated lasers and waveguides, including ring lasers with a variety of cavity configurations such as those described in U.S. Pat. No. 5,132,983 and in copending U.S. patent application Ser. No. 09/918,544, filed Aug. 1, 2001, the disclosures of which are hereby incorporated herein by reference. The development of these technologies and the capability of producing a wide range of laser and waveguide structures expands the prospective applications for integrated optical devices, and adds the attractiveness of greater manufacturability and reduced cost.
Optical couplers are conventionally used to couple light to and from integrated optical laser and waveguide devices; however, easy and efficient techniques for coupling such devices with external components such as optical fibers are not available. Although optical couplers of various designs have been developed, there is great difficulty in aligning such couplers with integrated optical devices and with external optical components such as optical fibers, and the resulting low yield produces high costs for such devices. Accordingly, a monolithic optical coupler having an arbitrary three-dimensional pattern would be very desirable, for it would enable cost effective, high yield fabrication of integrated optical components and their couplers to enable coupling of the devices with external components such as optical fibers.
In accordance with the present invention, three-dimensional structures of arbitrary shape are fabricated on the surface of a substrate through a series of processing steps which form a monolithic structure by fabricating it in successive layers. These layers preferably are formed from a lithographically definable material such as conducting polymers, resist materials, or the like. For convenience, the following description will refer to layers formed of photoresist materials, but it will be understood that such other materials can also be used. Thus, for example, in accordance with a preferred form of the invention, a first layer of a photoresist material is spun onto a substrate surface and is exposed to a desired pattern corresponding to the shape of the final structure at a corresponding level in the structure. The first layer is not developed after exposure; instead a second layer of photoresist material is deposited on top of the first layer and is also exposed to a pattern at least partially vertically aligned with the first pattern. Subsequent layers are spun onto the top surfaces of prior layers and additional aligned patterns are exposed. If desired, a barrier layer may be provided between successive layers to prevent intermixing. Upon completion of the successive vertically aligned layers defining the structures, the layers are all developed at the same time. This removes the exposed material (in the case of a positive photoresist) leaving the unexposed material behind to form a three-dimensional structure having levels corresponding to the exposed patterns. In the case of a negative photoresist material, the exposed material forms the structure.
The foregoing process utilizes either a positive or negative photoresist material, but in a modification of the process, the three-dimensional structure can be fabricated using some positive photosensitive materials and some negative photosensitive materials. The exposure of the photoresist materials preferably is done photolithographically, allowing a wide range of shapes and configurations. It will be understood, however, that e-beam, x-ray or other forms of radiation may be used to expose corresponding resist or other lithographically definable layers using the same layering technique disclosed herein.
The fabrication techniques of the invention may be used to produce optical couplers, gratings, and other multilayer devices of arbitrary shape for use with optical systems, in integrated circuit systems, and the like, where multilayered structures are needed.